Dear Giacomo,
If you want to simulate amorphous silica, you have to equilibrate the
liquid at high temperature before you quench it down slowly to room
temperature. If you anneal quartz up to 1000 K, you will not see
anything happening because, unfortunately, the quartz will not melt at
these temperatures in the simulation time you can afford. This is also
due to the small size of the simulation box.
There is a large number of papers on the subject. I have myself worked a
lot on amorphous silicates.
In order to create amorphous silica, there are mainly 2 approaches:
1/ you equilibrate a liquid at 3000 K and you anneal it (slowly !) to
300K using ab initio MD
2/ you equilibrate a liquid at 3000 K and you anneal it (as slow as
possible !) to 300K using CLASSICAL MD with a reasonable potential, then
you equilibrate the glass at 300 K using ab initio MD... This works in
most cases and will help you saving CPU time.
See these references for instance. These works were not done with SIESTA
but it would be similar, I guess:
1) "Model of silica glass from combined classical and ab initio
molecular dynamics \linebreak simulations" M. Benoit, S. Ispas, P. Jund
and R. Jullien, Euro. Phys. J. B 13, 631 (2000).
2) "The vibrational dynamics of vitreous silica: Classical force fields
vs. first-principles" M. Benoit and W. Kob, Europhys. Lett. 60, 269 (2002).
3) J. Sarnthein, A. Pasquarello, R. Car, Phys. Rev. Letters {\bf 74}
4682 (1995); J. Sarnthein, A. Pasquarello, R. Car, Phys. Rev. B {\bf 52}
12 690 (1995).
Good luck,
Magali
Giacomo Giorgi a écrit :
Dear All,
I would like to simulate a-SiO2 starting from quartz. I was
thinking to perform an annealing from T=300 K to 1000 K and then to
"freeze" the system from 1000K to 300K. I am honestly not too
experienced in this field, so if there is someone able to give me
useful hints I would appreciate it a lot.
Thanks,
Giacomo
